a) Field of the Invention
The present invention relates to an electronic imaging device for photographic cameras, etc. which is equipped with a photographic optical system comprising, in the order from the object side, a photographic lens system, a thin half mirror or transparent plate obliquely intersecting with the optical axis of the photographic lens system, and an electronic image pickup means which has a light receiving surface on which rectangular picture elements ate arranged regularly in a lattice pattern and arranged substantially perpendicular to the optical axis.
b) Description of the Prior Art
One of the greatest features of the camera using electronic image pickup elements lies in that it has a small imaging surface on the electronic image pickup element comprised therein and is compatible with photographic lens systems having high vari-focal ratios. The camera of this type uses not only the electronic view type finder but also the optical type finder which can provide images of high quality. As a finder for use with a photographic optical system having a high vari-focal ratio, it is adequate from the technical viewpoint to select the TTL type since it permits easy correction of the deviation between a photographing range and an observation range. For using the TTL type finder in combination with the photographic optical system, rays are led from the photographic optical system to the side of the finder by using mirrors, conventionally by using the so-called quick return mirror which is placed out of the photographing optical path only during the moment of photographing. The camera using the electronic image pickup element has another feature that it permits high-speed successive photographing operation, which is remarkably hindered by the quick return mirror having a slow operation speed. It is therefore preferable to use a half mirror in the camera using the electronic image pickup element. The camera using the electronic image pickup element is compatible with two types of half mirrors: one consisting of a surface coated with a reflective film sandwiched between two glass blocks, and the other composed of a glass plate coated with a reflective film. The former type has a high degree of polarization and is apt to produce problems in light measurement or image formation. In order to lessen these problems, it is necessary to reduce reflectance of the former type of half mirror as much as possible, or minimize the angle formed between the normal to the reflecting surface and the optical axis thereof. Accordingly, it is preferable to use the latter type of half mirror which has a lower degree of polarization with the camera using the electronic image pickup element. Further, it is possible to use a thin transparent plate which is designed so as to totally reflect a fraction of rays in place of the thin half mirror. When the half mirror or transparent plate is used, however, an image formed by the transmitted light is effected by astigmatic difference. That is to say, a certain deviation is produced between the imaging point of the components in the direction comprised in the plane including the normal to the reflecting surface and the optical axis (meridional surface) and that of the components in the direction included in the plane perpendicular to the meridional surface (sagittal surface). Since the deviation is nearly proportional to thickness of the half mirror or the transparent plate, this mirror or plate must be made of a glass plate or a plate of synthetic resin, etc. as thin as possible, but the astigmatic difference may be negligible when high resolution is demanded.